#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/socket.h>

#define BUF_SIZE 30

void error_handling(char *message);

int main(int argc, char *argv[])
{
    int serv_sock;
    char message[BUF_SIZE];
    int str_len;
    socklen_t clnt_adr_sz;

    struct sockaddr_in serv_adr, clnt_adr;
    if(argc!=2) {
        printf("Usage : %s <port>\n", argv[0]);
        exit(1);
    }

    serv_sock=socket(PF_INET, SOCK_DGRAM, 0);
    if(serv_sock==-1)
        error_handling("UDP socket creation error");

    memset(&serv_adr, 0, sizeof(serv_adr));
    serv_adr.sin_family=AF_INET;
    serv_adr.sin_addr.s_addr=htonl(INADDR_ANY);
    serv_adr.sin_port=htons(atoi(argv[1]));

    if(bind(serv_sock, (struct sockaddr*)&serv_adr, sizeof(serv_adr))==-1)
        error_handling("bind() error");

    while(1) 
    {
        clnt_adr_sz=sizeof(clnt_adr);
        str_len=recvfrom(serv_sock, message, BUF_SIZE, 0, 
                         (struct sockaddr*)&clnt_adr, &clnt_adr_sz);
        sendto(serv_sock, message, str_len, 0, 
               (struct sockaddr*)&clnt_adr, sizeof(clnt_adr));
    }
    close(serv_sock);
    return 0;
}

void error_handling(char *message)
{
    fputs(message, stderr);
    fputc('\n', stderr);
    exit(1);
}
